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What happens to soil when you apply N?

Everything you read on the Internet isn’t true. That’s probably not a shocker. Guess what? Everything you hear at the coffee shop or hanging around the office of your local ag retailer chatting with other farmers may not be true either.

May 11, 2011

What happens to soil when you apply N?

By Tom J. Bechman

Everything you read on the Internet isn’t true. That’s probably not a shocker. Guess what? Everything you hear at the coffee shop or hanging around the office of your local ag retailer chatting with other farmers may not be true either.

You don’t have to go to very many coffee shop discussions or loafing sessions to hear this one: “Anhydrous ammonia hurts my ground — I just know it does.” There are legitimate reasons not to choose anhydrous ammonia, such as safety of application, or how it fits with your tillage system. But hurting and destroying your ground isn’t a legitimate reason.

“As anhydrous ammonia is applied in a knife-track, it reduces bacteria and fungi populations in the zone surrounding the application,” says Danny Greene, a certified crop adviser, of Greene Consulting Inc., Franklin.

“However, it’s been shown that these populations recover after about a month’s time, depending upon application timing and soil conditions,” Greene says.

Impact zone

The zone of influence is about 2.5 to 3.5 inches in diameter from the tip point where anhydrous is injected into the soil, says Jim Camberato, a Purdue University Extension soil specialist. He confirms that bacteria and fungi within that zone will be wiped out, and so will an earthworm if he’s in the way of the knife.

The same thing happens if 28% N is injected, although the kill zone is not as large, the soil specialist notes. Typically, the zone where bacteria and fungi will be destroyed is about three-quarters to 1 inch in diameter at the tip point, he notes.

The irony is that while some helpful bacteria are destroyed, the larger kill zone with anhydrous actually prevents denitrification bacteria from growing back rapidly. They’re the ones that contribute to N losses. These bacteria actually return faster where liquid N was injected, because the impact zone wasn’t as large in the first place.

“The other factor is that 28% N is already 25% in the nitrate form when it’s applied,” Camberato says. “Anhydrous must be converted to that form by bacteria before losses begin. The fact that bacteria don’t grow back as quickly where anhydrous was applied because the impact zone was larger can actually be an advantage if you’re worried about losing nitrogen.”

Consider economics

Some may consider ammonium sulfate as a source to get around problems associated with anhydrous ammonia, such as safety concerns. If you do so, make sure you keep a close watch on soil pH, Camberato says.

Ammonium sulfate is up to three times as acidic as 28% N or anhydrous ammonia. Figure that it takes 5 pounds of ag lime to neutralize each pound of N applied by ammonium sulfate, and only 2 pounds of lime per pound of actual N supplied by liquid N or anhydrous ammonia.

“If you look at the cost effectiveness of anhydrous ammonia compared to the other options, anhydrous ammonia often tops the others,” Greene concludes. “For this reason I wouldn’t be too quick to mark anhydrous off my list.”

This article published in the March, 2011 editionof INDIANA PRAIRIE FARMER.